Because stupid commentator (in the 90s) always want to hype things and sell that the present players/present era are the best thing since sliced bread.
I laughted out loud in the 90s when I listened to them so many times saying that Sampras was probably the GOAT, and now they do exactly the same thing with Federer.
It is impossible to compare numbers from different eras, because then you'd say that Sampras was "better" than Lendl or McEnroe, or you'd say that Federer is "better" than Lendl or Borg. And it doesn't make any sense, because you are talking about players that played in different eras, against different players, on different conditions.
Nobody has a clue about how many GS would Federer have won in the 70s (had he been born 40 years early), it could be 20 GS or it could be 5 GS, nobody knows.
Nobody has a clue about how many GS (or YE nš1) would Lendl (or Borg, or Laver) have won in the current era (had anyone of them been born some decades later). Nobody knows.
We can, at best, guess that the best one or two best ones players from one era would be great in any other era, but even that we can not be absolutely sure, and we don't know what numbers would they get in any other era (conditions are not the same now than in the past, today fewer players win GS tournaments and because of that those few ones win a higher nš of GS than in the past when totally polarized conditions and different playing styles meant more different GS champions and obviously less players winning 10+ GS/lower nš of GS for the great ones of previous eras).
Sampras was the best player of his era, the best thing a tennis player can be, just like Pancho Gonzales was the best player from a given era, Lendl was the best player from a given era, Federer is the best player from the current era,...
Unless somebody creates an "alternative universes computer simulation", trying to compare players or numbers from different tennis eras is senseless.